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Battery novices often brag
about miracle batteries that offer very high energy densities, deliver
1000 charge/discharge cycles and are paper-thin. These attributes are
indeed achievable but not on one and the same battery pack.

A certain battery may be
designed for small size and long runtime, but this pack has a limited
cycle life. Another battery may be built for durability but is big and
bulky. A third pack may have high energy density and long durability but
this version is too expensive for the consumer.

Battery manufacturers are
aware of customer needs and offer packs that best suit the application.
The mobile phone industry is an example of this clever adaptation. Here,
small size and high energy density reign in favor of longevity. Short
service life is not an issue because a device is often replaced before
the battery is worn out.

Let's examine various
battery designs, starting with nickel-metal-hydride. The cylindrical
nickel-metal-hydride for commercial use offers a mid-range energy
density of about 80Wh/kg and delivers roughly 400 cycles. The prismatic
nickel-metal-hydride, a battery that is made for slim geometry,
compromises on energy density and cycle count. This battery is rated at
a moderate 60Wh/kg and offers around 300 cycles. Highly durable
nickel-metal-hydride for industrial use are packaged in cylindrical
cells, provide a modest 70Wh/kg but last for about 1000 cycles.

Similarly, lithium-ion
batteries can be produced with various energy densities. Packing more
energy into a cell compromises safety. While commercial lithium-ion
batteries are safe, super-high capacity lithium-ion for defense
applications are, for safety reasons, not approved for the public at
large.

Below is a summary of the
strength and limitations of today's popular battery systems. Although
energy density is paramount, other important attributes are service
life, load characteristics, maintenance requirements, self-discharge and
operational costs. Since nickel-cadmium remains a standard against which
batteries are compared, we evaluate alternative chemistries against this
classic battery type.

-Nickel-cadmium - mature
but has moderate energy density. nickel-cadmium is used where long life,
high discharge rate and extended temperature range is important. Main
applications are two-way radios, biomedical equipment and power tools.
nickel-cadmium contains toxic metals.

-Nickel-metal-hydride - has
a higher energy density compared to nickel-cadmium at the expense of
reduced cycle life. There are no toxic metals. Applications include
mobile phones and laptop computers.

-Lead-acid - most
economical for larger power applications where weight is of little
concern. Lead-acid is the preferred choice for hospital equipment,
wheelchairs, emergency lighting and UPS systems.

-Lithium-ion-polymer -
Similar to lithium-ion, this system enables slim geometry and simple
packaging at the expense of higher cost per watt/hours. Main
applications are cell phones.

-Reusable Alkaline - Its
limited cycle life and low load current is compensated by long shelf
life, making this battery ideal for portable entertainment devices and
flashlights.
Table 1 summarizes the characteristics of the common batteries. The
figures are based on average ratings at time of publication. Note that
nickel-cadmium has the shortest charge time, delivers the highest load
current and offers the lowest overall cost-per-cycle but needs regular
maintenance.

Nickel-cadmium

Nickel-metal-hydride

Lead-acid

Lithium-ion

Lithium-ion-polymer

Reusable Alkaline

Gravimetric Energy
Density (wh/kg)

45-80

60-120

30-50

110-160

100-130

80
(initial)

Internal
Resistance (includes peripheral circuits) in mΩ

100 to
200*1
6V pack

200 to
300*1
6V pack

<100*1
12V pack

150 to
250*1
7.2V pack

200 to
300*1
7.2V pack

200 to
2000*1
6V pack

Cycle Life
(to 80% of initial capacity)

1500*2

300 to
500*2, 3

200 to
300*2

300 to
500*3

300 to
500

50*3
(to 50% capacity)

Fast Charge Time

1h
typical

2 to
4h

8 to
16h

2 to 4h

2 to
4h

2 to
3h

Overcharge
Tolerance

moderate

low

high

very
low

low

moderate

Self-discharge
/Month
(room temperature)

20%*4

30%*4

5%

10%*5

-10%*5

0.3%

Cell Voltage
(nominal)

1.25V*5

1.25V*5

2V

3.6V

3.6V

1.5V

Load Current
peak best result

20C
1C

5C
0.5C or lower

5C*7
0.2C

>2C
1C or lower

>2C
1C or lower

0.5C
0.2C or lower

Operating
temperature*8
(discharge only)

-40 to
60℃

-20 to
60℃

-20 to
60℃

-20 to
60℃

0 to
60℃

0 to
65℃

Maintenance
Requirement

30 to
60 days

60 to
90 days

3 to 6
months*9

not
required

not
required

not
required

Typical Battery
Cost*10
(US$, reference only)

$50
(7.2V)

$60
(7.2V)

$25
(6V)

$100
(7.2V)

$100
(7.2V)

$5
(9V)

Cost per Cycle
(US$)*11

$0.04

$0.12

$0.10

$0.14

$0.29

$0.10
to 0.50

Commercial use
since

1950

1990

1970

1991

1999

1992

Table 1: Characteristics of
commonly used rechargeable batteries.

*1) Internal resistance of a
battery pack varies with cell rating, type of protection circuit and
number of cells. Protection circuit of lithium-ion and
lithium-ion-polymer adds about 100mW.
*2) Cycle life is based on battery receiving regular maintenance. Failing
to apply periodic full discharge cycles may reduce the cycle life by a
factor of three.
*3) Cycle life is based on the depth of discharge. Shallow discharges
provide more cycles than deep discharges.
*4) The discharge is highest immediately after charge, and then tapers
off. The capacity of nickel-cadmium decreases 10% in the first 24h, then
declines to about 10% every 30 days thereafter. Self-discharge increases
with higher temperature.
*5) Internal protection circuits typically consume 3% of the stored
energy per month.
*6) 1.25V is the open cell voltage. 1.2V is the commonly used as a method
of rating.
*7) Capable of high current pulses.
*8) Applies to discharge only; charge temperature range is more confined.
*9) Maintenance may be in the form of 'equalizing' or 'topping' charge.
*10) Cost of battery for commercially available portable devices.
*11) Derived from the battery price divided by cycle life. Does not
include the cost of electricity and chargers.

In subsequent columns I
will describe the strength and limitation of each chemistry in more
detail. We will examine charging techniques and explore methods to get
the most of these batteries.